Bias calculation, bias compensation, and bias table editing methods for HDD, recording media storing computer programs for executing the same, and HDD using the same

ABSTRACT

In a hard disk drive (HDD), biases are measured across all zones of a disk, and nonlinear bias components are extracted. Upon a change in operational condition of the HDD, biases are measured at two reference positions on the disk so as to estimate a corresponding change in a linear bias component across all zones of the disk. The bias at a given position of the disk is then calculated based on the estimated linear bias component and the extracted nonlinear bias component.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to hard disk drives (HDDs), andmore particularly, the present invention relates to bias compensationfor servo control of HDDs.

A claim of priority is made to Korean Patent Application No.10-2005-0082440, filed on Sep. 5, 2005, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

2. Description of the Related Art

A hard disk drive (HDD) is an auxiliary storage unit of a computersystem which is capable of accessing bulk data at high speed bymagnetically reading/writing data from/on a rotating magnetic disk. Thatis, data is stored in concentric tracks formed on the magnetic disk, andthe tracks are accessed by a magnetic head for writing or reading dataon or from the magnetic disk. The operation of moving the magnetic headacross the magnetic disk and positioning the head above one of thetracks is referred to as a “track seek” operation.

In the track seek operation, various disturbances affecting the magnetichead is referred to as a “bias”. Bias forces affecting the magnetic headmay include, for example, atmospheric pressure against a side surface ofthe head resulting from air flow generated when the disk is rotating,stresses caused by a flexible cable connected to the head, and bearingaxis characteristics of a voice coil motor (VCM).

It is also known that the bias varies with a position, a movingdistance, and a moving direction of the magnetic head. Therefore, forexample, since an initial driving force of a voice coil varies with theposition and moving direction of the magnetic head in a track seekoperation, and since an initial moving speed of the magnetic head varieswith the moving distance, the bias is compensated for according to theposition, moving distance, and moving direction of the magnetic head.Methods of compensating for the bias are disclosed in Korean RegisteredPatent Nos. 403,037 and 260,411 and U.S. Pat. Nos. 5,773,948 and6,738,220.

According to a conventional bias compensation method, a disk is dividedinto a plurality of zones from the inner circumference to the outercircumference, a bias is measured in each zone and stored in a biastable, and the bias is compensated for by referring to the bias tablewhen a track seek is performed.

Since bias can vary with the operational environment, it is preferablethat all biases corresponding to all zones of the disk in the bias tablebe updated with changes in operational environment. However, thecalculations needed to update the bias table in this manner are complexand require long computation times.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a method of calculatinga bias applied to a head of a hard disk drive (HDD) is provided. Themethod includes measuring biases across all zones of a disk, extractinga nonlinear bias component which is nonlinearly varied across all zonesof the disk based on the measured biases, measuring biases at tworeference positions on the disk according to a change of an operationalcondition of the HDD, estimating a linear bias component which islinearly varied across all zones of the disk based on the biasesmeasured at the two reference positions, and calculating a bias at aposition of the disk according to the change of the operationalcondition of the HDD based on the estimated linear bias component andthe extracted nonlinear bias component.

According to another aspect of the present invention, a method ofcompensating for a bias applied to a head of a hard disk drive (HDD) isprovided. The method includes measuring biases across all zones of adisk and extracting a linear bias component which is linearly changedacross all zones of the disk and a nonlinear bias component which isnonlinearly changed across all zones of the disk based on the measuredbiases, storing the linear bias component and the nonlinear biascomponent in a bias table, and compensating for a bias by referring tothe bias table when the HDD is turned on.

According to another aspect of the present invention, a method ofediting a bias table for compensating for a bias applied to a head of ahard disk drive (HDD) is provided. The method includes measuring biasesacross all zones of a disk and extracting a linear bias component whichis linearly changed across all zones of the disk and a nonlinear biascomponent which is nonlinearly changed across all zones of the diskbased on the measured biases, and editing a bias table which stores thelinear bias component and the nonlinear bias component.

According to another aspect of the present invention, a computerreadable recording medium storing a program for executing a method ofcalculating a bias applied to a head of a hard disk drive (HDD), wherethe method includes measuring biases across all zones of a disk,extracting a nonlinear bias component which is nonlinearly varied acrossall zones of the disk based on the measured biases, measuring biases attwo reference positions on the disk according to a change of anoperational condition of the HDD, estimating a linear bias componentwhich is linearly varied across all zones of the disk based on thebiases measured at the two reference positions, and calculating a biasat a position of the disk according to the change of the operationalcondition of the HDD based on the estimated linear bias component andthe extracted nonlinear bias component.

According to another aspect of the present invention, a computerreadable recording medium storing a program for executing a method ofcompensating for a bias applied to a head of a hard disk drive (HDD) isprovided, where the method includes measuring biases across all zones ofa disk and extracting a linear bias component which is linearly changedacross all zones of the disk and a nonlinear bias component which isnonlinearly changed across all zones of the disk based on the measuredbiases, storing the linear bias component and the nonlinear biascomponent in a bias table, and compensating for a bias by referring tothe bias table when the HDD is turned on.

According to another aspect of the present invention, a computerreadable recording medium storing a program for executing a method ofediting a bias table for compensating for a bias applied to a head of ahard disk drive (HDD) is provided, where the method includes measuringbiases across all zones of a disk and extracting a linear bias componentwhich is linearly changed across all zones of the disk and a nonlinearbias component which is nonlinearly changed across all zones of the diskbased on the measured biases, and editing a bias table which stores thelinear bias component and the nonlinear bias component.

According to another aspect of the present invention, a hard disk drive(HDD) is provided which includes a disk which stores information, aspindle motor which rotates the disk, a head which records informationon the disk and which reads information from the disk, a voice coilmotor (VCM) which moves the head, a memory which stores a bias table,and a controller which measures a bias in a track and which compensatesfor the bias in a track seek mode by referring to the bias table storedin the memory. The memory stores a linear bias component which islinearly changed across all zones of the disk and a nonlinear biascomponent which is nonlinearly changed across all zones of the disk, andthe controller compensates for the bias by referring to the bias tablewhen the HDD is turned on, measures biases at reference positions on thedisk when an operational condition of the HDD is changed, estimates alinear bias component changed across all zones of the disk based onbiases measured at the reference positions, and updates the linear biascomponent stored in the bias table to the estimated linear biascomponent.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects and features of the present invention willbecome readily apparent from the detailed description that follows, withreference to the accompanying drawings, in which:

FIG. 1 illustrates bias profiles measured in an HDD;

FIG. 2 illustrates zones divided on a disk in an HDD;

FIGS. 3A-3B and 4A-4B are diagrams for schematically explainingnonlinear bias components;

FIG. 5 is a diagram for use in explaining the manner in which linear andnonlinear bias components are obtained from a bias profile;

FIG. 6 is a diagram illustrating bias profiles with respect to usagetime of an HDD and linear components corresponding to the measured biasprofiles;

FIG. 7 is a flowchart of a bias calculation method for an HDD accordingto an embodiment of the present invention;

FIG. 8 schematically illustrates a method of estimating a linear biascomponent in the bias calculation method illustrated in FIG. 7 accordingto an embodiment of the present invention;

FIG. 9 illustrates an effect of a bias calculation method according toan embodiment of the present invention;

FIG. 10 is a flowchart of a bias compensation method according to anembodiment of the present invention;

FIG. 11 is a flowchart of a method of editing a bias table according toan embodiment of the present invention;

FIG. 12 is a flowchart of a method of editing a bias table according toanother embodiment of the present invention;

FIG. 13 is a schematic plan view of an HDD according to an embodiment ofthe present invention;

FIG. 14 is a block diagram of an electrical circuit of the HDDillustrated in FIG. 13 according to an embodiment of the presentinvention; and

FIG. 15 is a block diagram of a track seek servo control system executedby a controller illustrated in FIG. 14 according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates bias profiles measured in an HDD. In graph of FIG. 1,the horizontal axis denotes a head position (track number) on a disk,i.e., a distance from an inner circumference of the disk, and thevertical axis denotes the magnitude of a bias torque (in arbitraryunits). As illustrated in FIG. 1, a bias profile varies according to ahead position on the disk.

The bias profiles illustrated in FIG. 1 represent different usage timesof the HDD. For example, the curve at the top of the graph is a biasprofile immediately after the HDD is turned on, and curve at the bottomis a bias profile after the HDD has been operated for the longest timerelative to the other curves. As suggested by FIG. 1, the bias convergesto certain values with the passage of time.

The bias varies not only with the head position and usage time asillustrated in FIG. 1, but also with a moving direction of the head,i.e., depending on whether the head moves radially inward or outward.

FIG. 2 illustrates zones divided on a disk in an HDD. It is known thatthe HDD is managed by dividing an area from the inner circumference ofthe disk to the outer circumference into a plurality zones (zone 0 tozone N). A track pitch, the number of sectors per track, and write/readparameters are set differently for each zone. A bias is also separatelymeasured and compensated for each zone.

According to a conventional bias compensation method, a bias tablecontaining biases obtained by measurement or estimation is stored in amemory, and the bias table is referred to when a track seek isperformed. In addition, the bias table is updated by re-measuring orre-calculating biases every time a change of an operational condition,such as lapse of a usage time or a change of an operational temperature,occurs.

However, when all biases of all zones of the disk in the bias table areupdated, it long computation time is needed since a bias should be newlyobtained for each zone. In addition, if a bias varies nonlinearly, ittakes a much longer time since an operation of applying a proportionalconstant according to a head position on the disk to the bias isnecessary.

In a bias compensation method according to an embodiment of the presentinvention, a bias is compensated for by dividing the bias into a linearcomponent, which is linearly changed across all zones of a disk, and anonlinear component, which is nonlinearly changed across all zones ofthe disk, but only the linear component is compensated for when anoperational condition is changed.

Based on experimental results, although both a linear component and anonlinear component of a bias are changed when an operational conditionis changed, the degree of change of the linear bias component isconsiderably larger than that of the nonlinear bias component. Since thenonlinear bias component variation is relatively small it can beneglected during bias compensation.

Thus, a bias can be compensated for by obtaining a nonlinear componentin advance and estimating only a linear component when an operationalcondition is changed. Since the linear bias component varies linearlyacross all zones of a disk, the linear bias component can be simply andquickly estimated by referring to biases measured at two arbitrarypositions on the disk.

It is known that a significant source of bias is the stress of aflexible printed circuit board (FPCB) connecting a PCB in which circuitparts of the HDD are installed to a head, combined with an actuator anda mechanical shape of the actuator and the head.

The stress of the FPCB affects movement of the head since the FPCBexpands or contracts when the actuator operates, and the shape of theactuator and the head also affects movement of the head in combinationwith air flow generated when a disk is rotated.

Although the stress of the FPCB varies with a position of the actuator(i.e., a head position on the disk) and a moving direction of theactuator (i.e., a moving direction of the head), the most significantfactor is an operational temperature of the HDD. That is, when theoperational temperature of the HDD varies, the stress of the FPCBvaries, and accordingly, the bias applied to the head varies. Theoperational temperature of the HDD is strongly related to the usage timeof the HDD.

A bias caused by the mechanical shape of the actuator and the head isaffected by the head position on the disk and the moving direction ofthe head rather than the operational temperature of the HDD.

Based on the above description, a bias of the HDD can be divided into acomponent considerably affected by the usage time and the operationaltemperature and a component considerably not affected by them. It can beconsidered that the component considerably affected by the usage timeand the operational temperature is scarcely affected by the mechanicalshape of the actuator and the head, the head position on the disk, andthe moving direction of the head and is linearly changed across allzones of the disk.

It can be also considered that the component considerably not affectedby the usage time and the operational temperature is affected by themechanical shape of the actuator and the head, the head position on thedisk, and the moving direction of the head and is nonlinearly changedacross all zones of the disk, but the level of the affection is notconsiderably changed with usage time and the operational temperature.

FIGS. 3A-3B and 4A-4B are diagrams for schematically explainingnonlinear bias components.

FIG. 3A illustrates a plurality of bias profiles measured atrespectively different usage times while the head moves from the innercircumference to the outer circumference (i.e., radially outward), andFIG. 4A illustrates a plurality of bias profiles measured while the headmoves from the outer circumference to the inner circumference (i.e.,radially inward). In each of these figures, the horizontal axis denotesa head position (track number) on a disk, and the vertical axis denotesthe magnitude of a bias torque (in arbitrary units).The initial biasprofile in each figure was measured at the time when the HDD isinitially used, i.e., when the HDD is turned on, and the remaining biasprofiles were measured at constant time intervals thereafter.

The bias profiles illustrated in FIGS. 3A and 4A include nonlinear andlinear components. FIGS. 3B and 4B illustrate the nonlinear biascomponents corresponding to the bias profiles illustrated in FIGS. 3Aand 4A. As illustrated in FIGS. 3B and 4B, very small differences existbetween the nonlinear bias components according to usage time of theHDD.

Since the nonlinear bias components have very small differences with theusage time, a sufficient bias compensation performance can be obtainedeven though the nonlinear component is not updated with usage time.

FIG. 5 is a diagram for schematically illustrating the manner in whichlinear and nonlinear bias components are obtained from a bias profile.In FIG. 5, the horizontal axis denotes a head position (track number) ona disk, and the vertical axis denotes a bias compensation value (inarbitrary units). A bias profile 502 and a straight line 504 whichapproximates the bias profile 502 are shown in FIG. 5. Referring to FIG.5, the bias profile 502 can be represented using the straight line 504and a difference between the bias profile 502 and the straight line 504.

That is, a bias F is represented by F=f(p)+0+a1p where p denotes aposition on the disk, f(p) denotes a difference value, a0 denotes anintercept of the straight line 504, and a1 denotes a slope of thestraight line 504.

In the above equation, f(p) is a nonlinear component of the bias, anda1+a1p is a linear component of the bias.

To increase accuracy, the straight line 504 is set so that thedifference value between the bias profile 502 and the straight line 504is minimized. That is, the straight line 504 is set so that a total sumof root squares of difference values at every position on the disk isminimized.

FIG. 6 is a diagram illustrating bias profiles measured with respect toa usage time of an HDD and linear components corresponding to themeasured bias profiles. In FIG. 6, the horizontal axis denotes a headposition (track number) on a disk, and the vertical axis denotes themagnitude of a bias torque (in arbitrary units). Bias profiles 602 athrough 602 d of FIG. 6 are measured with respect to the usage time ofthe HDD, and straight lines 604 a through 604 d are linear componentscorresponding to the bias profiles 602 a through 602 d.

Referring to FIG. 6, as the usage time of the HDD increases, the biasprofiles 602 a through 602 d show large differences, in contrast withthe nonlinear bias components which show very little differences aspreviously discussed in connection with FIGS. 3B and 4B.

Thus, a bias at an arbitrary position can be calculated by obtainingnonlinear bias components in advance, estimating linear bias componentsaccording to variations of an operational condition, i.e., variations ofa usage time and an operational temperature, and adding the nonlinearbias components and the linear bias components.

FIG. 7 is a flowchart of a bias calculation method for an HDD accordingto an embodiment of the present invention.

Referring to FIG. 7, a bias profile is obtained by measuring biasesacross all zones of a disk in operation S702.

A nonlinear bias component which is nonlinearly changed across all zonesof the disk is extracted based on the measured bias profile in operationS704. The extracting of the nonlinear bias component was describedpreviously in connection with FIG. 5. Operations S702 and S704 may beperformed in a manufacturing process of the HDD, in particular, duringburn-in test process. The obtained nonlinear bias component is stored ina bias table, and the bias table is stored in a maintenance cylinder ofthe disk.

Biases are measured at two reference positions on the disk in operationS706. The two reference positions may be arbitrarily selected.

A linear bias component which is linearly changed across all zones ofthe disk is estimated based on the measured biases in operation S708.

FIG. 8 schematically illustrates a method of estimating the linear biascomponent in the bias calculation method illustrated in FIG. 7. In FIG.8, the horizontal axis denotes a head position (track number) on a disk,and the vertical axis denotes a bias compensation value (in arbitraryunits).

As illustrated in FIG. 8, the linear bias component is represented by astraight line estimation 802 of a bias profile 801 extending between tworeference positions P_a and P_b on the disk. Again, the two referencepositions P_a and P_b may be arbitrarily selected.

In operation S710, a bias at a desired position on the disk iscalculated based on the linear bias component estimated in operationS708 and the nonlinear bias component extracted in operation S704.

FIG. 9 illustrates an effect of the bias calculation method according toan embodiment of the present invention. In FIG. 9, the horizontal axisdenotes a head position (track number) on a disk, and the vertical axisdenotes a bias compensation value (in arbitrary units). Also, in FIG. 9,reference numeral 902 denotes an initial bias profile, e.g., a biasprofile measured right after an HDD is turned on, reference numeral 904denotes a bias profile calculated using the bias calculation methodaccording to an embodiment of the present invention after a certaintime, e.g., 3 minutes, since the HDD has been turned on, and referencenumeral 906 denotes an actually measured bias profile corresponding tothe calculated bias profile 904. Only a small difference exists betweenthe bias profile 904 calculated using the bias calculation methodaccording to an embodiment of the present invention and the actuallymeasured bias profile 906.

Referring to a bias profile 912 obtained after the usage time isincreased a bit and bias profiles 914 and 916 calculated and measuredafter a certain time, again only a small difference exists between thecalculated bias profile 914 and the measured bias profile 916. This isbecause the bias converges as the usage time increases. Thus, the biascalculation method according to an embodiment of the present inventionis more accurate as the usage time increases.

FIG. 10 is a flowchart of a bias compensation method according to anembodiment of the present invention. Referring to FIG. 10, when an HDDis turned on, a bias table recorded in a maintenance cylinder of a diskis read and stored in a memory in operation S1002.

The bias table contains a nonlinear bias component and a linear biascomponent corresponding to an initial use. The manner of obtaining thenonlinear bias component and the linear bias component will be describedin detail later.

Bias compensation is performed by referring to the initial bias tablestored in the memory in operation S1004.

It is checked whether an error is larger than a predetermined value inthe bias compensation in operation S1006. That is, during normaltracking of the disk, an existing closed loop controller continuouslyestimates the bias using, for example, integral control to obtain zeroerror. The error in operation S1006 is the difference between thiscontinuously estimated bias and the value calculated in operation S1004.If the error is not larger than the predetermined value, the biascompensation is continuously performed through operation S1004.

A bias compensation value for the bias compensation is obtained byadding the nonlinear bias component and the linear bias componentcontained in the bias table. An equation of a straight line forrepresenting the linear bias component is stored in the bias table, anda linear component of a bias at a certain position is obtained bysubstituting a head position on the disk in the equation.

The nonlinear bias component is stored in the bias table according to ahead position on the disk. That is, a graph as illustrated in FIGS. 3Band 4B is stored in the bias table.

When the linear bias component at the certain position is obtained, abias at the certain position is obtained by adding the obtained linearbias component and a nonlinear bias component at the certain position.The bias compensation is performed using the obtained bias.

If the error is larger than the predetermined value, the bias table isupdated. When this happens, a bias profile to be used for the biascompensation is considerably different from a previously used biasprofile, and thus the linear bias component must be updated.

Biases are measured at two arbitrary positions, and a linear biascomponent is estimated by referring to the measured biases in operationS1008.

The linear bias component contained in the bias table stored in thememory is updated to the estimated linear bias component in operationS1010. Thereafter, the bias compensation is performed using the updatedbias table.

FIG. 11 is a flowchart of a method of editing a bias table according toan embodiment of the present invention.

Referring to FIG. 11, a bias profile is edited in operation S1102. Thebias profile is obtained by measuring biases across all zones of a diskwhile a head is being moved on the disk. Various methods of measuringthe biases can exist, and the biases are simply obtained by measuring aDC component of a driving current applied to a VCM during trackfollowing.

A linear bias component and a nonlinear bias component are obtained byreferring to the measured bias profile in operation 1104. How to obtainthe linear bias component and the nonlinear bias component has beendescribed in detail referring to FIG. 5.

The obtained linear bias component and nonlinear bias component arewritten in the bias table in operation S1106. The linear bias componentis stored as an equation of a straight line obtained by referring toFIG. 5, i.e., an equation whose intercept is a0 and whose slope is a1,and the nonlinear bias component is stored according to a head positionon the disk.

The bias table is stored in a maintenance cylinder of the disk or anonvolatile memory and thereafter used by an HDD. For example, the biastable is used for bias compensation in a track seek.

FIG. 12 is a flowchart of a method of editing a bias table according toanother embodiment of the present invention.

Referring to FIG. 12, bias profiles are edited in operation S1202. Thebias profiles are obtained by measuring biases across all zones of adisk while a head is being moved on the disk and measured according to ausage time of an HDD.

Nonlinear bias components for the respective bias profiles are obtainedin operation S1204.

A nonlinear bias component to be used for bias compensation iscalculated by averaging the nonlinear bias components for the respectivebias profiles in operation S1206.

A linear component of a bias profile obtained right after the HDD isturned on and the nonlinear bias component obtained in operation S1206are written in the bias table in operation S1208. The linear componentof the bias profile obtained right after the HDD is turned on is storedin the bias table because the bias table read from a maintenancecylinder and stored in a memory during an initial operation afterturning on the HDD is used for the bias compensation.

In the method illustrated in FIG. 12, the nonlinear bias component to beused for the bias compensation uses the nonlinear bias componentsobtained from the bias profiles measured according to lapse of the usagetime.

The bias table editing method illustrated in FIG. 11 or 12 may beperformed in a manufacturing process of the HDD, in particular, in aburn-in test process.

FIG. 13 is a schematic plan view of an HDD 10 according to an embodimentof the present invention. Referring to FIG. 13, the HDD 10 includes atleast one magnetic disk 12 rotated by a spindle motor 14. The HDD 10also includes a head 16 adjacently located to the surface of the disc12.

The head 16 can read or write information from or to the rotating disk12 by sensing a magnetic field formed on the disk 12 or magnetizing thedisk 12. Though a single head 16 is shown in FIG. 13, the head 16actually include a write head for magnetizing the disk 12 and a separateread head for sensing a magnetic field of the disk 12. The read head iscomposed of a magneto-resistive (MR) component.

The head 16 can be combined with a slider 20. The slider 20 generates anair bearing between the head 16 and the disk 12. The slider 20 iscombined with a head gimbal assembly (HGA) 22. The HGA 22 is attached toan actuator arm 24 having a voice coil 26. The voice coil 26 is locatedadjacently to a magnetic assembly 28 specifying a voice coil motor (VCM)30. A current supplied to the voice coil 26 generates a torque whichrotates the actuator arm 24 around a bearing assembly 32. The rotationof the actuator arm 24 moves the head 16 across the disk 12.

Information is typically stored in concentric tracks 34 of the disk 12.In general, each track 34 includes a plurality of sectors. Each sectorincludes a data field and an identification field. The identificationfield is composed of a gray code for identifying sectors and tracks(cylinders). The head 16 moves across the disk 12 to read or writeinformation from or in another track. In general, moving the head 16 toanother track is called a track seek routine.

FIG. 14 is a block diagram of an electrical circuit of the HDDillustrated in FIG. 13. Referring to FIG. 14, the HDD includes a disk12, a magnetic head 16, a pre-amplifier 210, a write/read (R/W) channel220, a buffer 230, a controller 240, a read only memory (ROM) 250A, arandom access memory (RAM) 250B, a host interface 260, a VCM driver 270.

Various commands and data used by the controller 240 to execute softwareroutines are stored in the ROM 250A. One of the software routines is aseek control routine of compensating for a bias by referring a biastable of the HDD and moving the head 16 from one track to another. Inaddition, equations for generating, for example, acceleration, velocity,and position trajectories of a sine waveform for a track seek are storedin the ROM 250A.

Information required for driving the HDD, which is read from the ROM250A or the disk 12 in an initial operation, is stored in the RAM 250B.In particular, the bias table to be referred to for bias compensation ina track seek is stored in the RAM 250B.

The controller 240 analyzes a command received from a host device (notshown) through the host interface 260 and executes a controlcorresponding to the analysis result. The controller 240 supplies acontrol signal to the VCM driver 270 to control activation of a VCM anda motion of the magnetic head 16.

A general operation of the HDD will now be described.

In a data read mode, the HDD amplifies an electrical signal sensed bythe read head of the magnetic head 16 from the disk 12 using thepre-amplifier 210. The RIW channel 220 amplifies the signal amplified bythe pre-amplifier 210 to a predetermined level by controlling a gainusing an automatic gain control circuit (not shown), encodes the analogsignal amplified to the predetermined level by the automatic gaincontrol circuit into a digital signal readable by the host device,converts the digital signal to a data stream, temporarily stores thestream data in the buffer 230, and transmits the stream data to the hostdevice through the host interface 260.

In a data write mode, the HDD receives data from the host device throughthe host interface 260, temporarily stores the received data in thebuffer 230, converts the data stored in the buffer 230 to a binary datastream suitable for a write channel using the R/W channel 220 bysequentially outputting the data stored in the buffer 230, and recordsthe binary data stream on the disk 12 through the magnetic head 16 usinga write current amplified by the pre-amplifier 210.

In a track seek operation, the controller 240 calculates a seek timeaccording to a seek distance, generates acceleration, velocity, andposition trajectories based on the calculated seek time, and executes aprocess of controlling a driving current of the VCM. Here, thecontroller 240 compensates for the driving current of the VCM breferring to the bias table stored in the RAM 250B.

FIG. 15 is a block diagram of an example of a track seek servo controlsystem including hardware and software executed by the controller 240illustrated in FIG. 14.

Referring collectively to FIGS. 13, 14 and 15, a seek trajectorygenerator 60 calculates a designed position x_(d)(n), a designedvelocity v_(d)(n), and designed acceleration a_(d)(n) of the head 16from an acceleration trajectory, and velocity and position trajectoriesobtained by integrating the acceleration trajectory of a sine wave everytime the head 16 reads a gray code recorded in a track 34.

A state estimator 62 performs a process of estimating a head motionstate variable value containing actual position and velocity informationof the head 16 from a received position error signal and a receivedcontrol signal uk using a state equation.

A first adder 64 subtracts an actual position x_(r)(n) from the designedposition x_(d)(n). A position control gain compensator 66 generates aposition compensation value by multiplying a position gain k_(p) forposition compensation by a difference between the actual positionx_(r)(n) and the designed position x_(d)(n) calculated by the firstadder 64.

A second adder 68 adds the designed velocity v_(d)(n) to the positioncompensation value generated by the position control gain compensator 66and then subtracts an actual velocity v_(r)(n) from the added result.

A velocity control gain compensator 70 generates a velocity compensationvalue by multiplying a velocity gain k_(r) for velocity compensation bythe value calculated by the second adder 68.

A third adder 72 generates a seek driving current control value uk(n) byadding the velocity compensation value to the designed accelerationa_(d)(n).

A bias force compensator 74 generates a seek driving current controlvalue obtained by compensating for a bias according to a head positionto the seek driving current control value uk(n) by referring to the biastable stored in the RAM 250B and applies the generated seek drivingcurrent control value to a VCM driver 76. The VCM driver 76 moves thehead 16 by rotating the VCM by supplying a current corresponding to thebias-compensated seek driving current control value to a voice coil of ahead disk assembly (HDA) 10.

Acceleration, velocity, position, and current trajectories according toa seek servo algorithm of a sine waveform can be obtained using Equation1.

$\begin{matrix}{{{x(t)} = {{K_{A}I_{M}{{\frac{T_{SK}}{2\pi}\left\lbrack {t - {\frac{T_{SK}}{2\pi}{\sin\left( {\frac{2\pi}{T_{SK}}t} \right)}}} \right\rbrack}.{v(t)}}} = {K_{A}I_{M}{\frac{T_{SK}}{2\pi}\left\lbrack {1 - {\cos\left( {\frac{2\pi}{T_{SK}}t} \right)}} \right\rbrack}}}}{{a(t)} = {K_{A}I_{M}{\sin\left( {\frac{2\pi}{T_{SK}}t} \right)}}}{{i(t)} = {I_{M}{\sin\left( {\frac{2\pi}{T_{SK}}t} \right)}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Here, T_(SK) denotes a seek time, I_(M) denotes a maximum currentsupplied to the voice coil, and K_(A) denotes an acceleration constant.

For a given seek distance X_(SK), a time t is equal to the seek timeT_(SK) and can be obtained using Equation 2.

$\begin{matrix}{X_{SK} = {{x\left( T_{SK} \right)} = {K_{A}I_{M}\frac{T_{SK}^{2}}{2\pi}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

From Equation 2, the required seek time T_(SK) for the given seekdistance X_(SK) can be obtained using Equation 3.

$\begin{matrix}{T_{SK} = \sqrt{\frac{2{\pi X}_{SK}}{K_{A}I_{M}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

According to Equation 3, a calculated seek trajectory of a sine wavespecified by the current I_(M) applied to the VCM is generated.

However, in reality, since a motion of the head 16 is disturbed by abias affecting the head 16, it is necessary to compensate for thedisturbance.

The controller 240 compensates for the current I_(M) applied to the VCMby referring to the bias table stored in the RAM 250B.

To do this, as described in FIG. 9, the controller 240 stores the biastable recorded in the maintenance cylinder of the disk 12 in the RAM250B when the HDD is turned on.

Thereafter, the controller 240 performs bias compensation by referringthe bias table stored in the RAM 250B in a track seek. If an error valueis larger than a predetermined value in the bias compensation, thecontroller 240 measures biases at two arbitrary positions on the disk12, estimates a linear bias component using the measured biases, andupdates a linear bias component stored in the RAM 250B to the estimatedlinear bias component.

When a track seek operation begins, the controller 240 compensates for abias by referring to the bias profile stored in the RAM 250B.

As described above, in a bias compensation method according to anembodiment of the present invention, since a bias at an arbitraryposition on a disk can be simply estimated using biases measured at tworeference positions on the disk when an operational condition of an HDDis changed, bias compensation is simply and quickly achieved.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. A method of calculating a bias applied to a head of a hard disk drive(HDD), the method comprising: measuring biases across all zones of adisk; extracting a nonlinear bias component which is nonlinearly variedacross all zones of the disk based on the measured biases, wherein thenonlinear bias component corresponds to a difference between themeasured bias at each zone of the disk and a linear approximation of thebias across all zones of the disk; measuring biases at two referencepositions on the disk according to a change of an operational conditionof the HDD; estimating a linear bias component which is linearly variedacross all zones of the disk based on the biases measured at the tworeference positions; and calculating a bias at a position of the diskaccording to the change of the operational condition of the HDD based onthe estimated linear bias component and the extracted nonlinear biascomponent.
 2. The method of claim 1, wherein the operational conditionis a usage time since the HDD has been turned on.
 3. The method of claim1, wherein the operational condition is an operational temperature ofthe HDD.
 4. The method of claim 1, wherein the measuring of the biasesacross all zones of the disk and the extraction of the nonlinear biascomponent are performed in a manufacturing process of the HDD.
 5. Themethod of claim 1, wherein the measuring of the biases across all zonesof the disk is performed right after the HDD is turned on.
 6. A methodof calculating a bias applied to a head of a hard disk drive (HDD), themethod comprising: measuring biases across all zones of a disk;extracting a nonlinear bias component which is nonlinearly varied acrossall zones of the disk based on the measured biases; measuring biases attwo reference positions on the disk according to a change of anoperational condition of the HDD; estimating a linear bias componentwhich is linearly varied across all zones of the disk based on thebiases measured at the two reference positions; and calculating a biasat a position of the disk according to the change of the operationalcondition of the HDD based on the estimated linear bias component andthe extracted nonlinear bias component, wherein the extraction of thenonlinear bias component comprises: setting a straight lineapproximating the biases across all zones of the disk; and extractingthe nonlinear bias component using differences between the biases acrossall zones of the disk and the straight line.
 7. The method of claim 6,wherein the straight line is set so that a total sum of the differencesbetween the biases across all zones of the disk and the straight line isminimized.
 8. The method of claim 7, wherein the straight line is set sothat a total sum of root squares of the differences between the biasesacross all zones of the disk and the straight line is minimized.
 9. Amethod of compensating for a bias applied to a head of a hard disk drive(HDD), the method comprising: measuring biases across all zones of adisk and extracting a linear bias component which is linearly changedacross all zones of the disk and a nonlinear bias component which isnonlinearly changed across all zones of the disk based on the measuredbiases, wherein the nonlinear bias component corresponds to a differencebetween the measured bias at each zone of the disk and a linearapproximation of the biases across all zones of the disk; storing thelinear bias component and the nonlinear bias component in a bias table;and compensating for a bias by referring to the bias table when the HDDis turned on.
 10. The method of claim 9, further comprising: measuringbiases at two reference positions on the disk when an operationalcondition of the HDD is changed; estimating a linear bias componentchanged across all zones of the disk based on the biases measured at thetwo positions; updating the linear bias component stored in the biastable to the estimated linear bias component; and compensating a bias byreferring to the updated bias table.
 11. A method of editing a biastable for compensating for a bias applied to a head of a hard disk drive(HDD), the method comprising: measuring biases across all zones of adisk and extracting a linear bias component which is linearly changedacross all zones of the disk and a nonlinear bias component which isnonlinearly changed across all zones of the disk based on the measuredbiases, wherein the nonlinear bias component corresponds to a differencebetween the measured bias at each zone of the disk and a linearapproximation of the biases across all zones of the disk; and editing abias table which stores the linear bias component and the nonlinear biascomponent.
 12. The method of claim 11, further comprising: measuringbiases at reference positions on the disk when an operational conditionof the HDD is changed; estimating a linear bias component changed acrossall zones of the disk based on biases measured at the referencepositions; and updating the linear bias component stored in the biastable to the estimated linear bias component.
 13. A computer readablerecording medium storing a program for executing a method of calculatinga bias applied to a head of a hard disk drive (HDD), the methodcomprising: measuring biases across all zones of a disk; extracting anonlinear bias component which is nonlinearly varied across all zones ofthe disk based on the measured biases, wherein the nonlinear biascomponent corresponds to a difference between the measured bias at eachzone of the disk and a linear approximation of the biases across allzones of the disk; measuring biases at two reference positions on thedisk according to a change of an operational condition of the HDD;estimating a linear bias component which is linearly varied across allzones of the disk based on the biases measured at the two referencepositions; and calculating a bias at a position of the disk according tothe change of the operational condition of the HDD based on theestimated linear bias component and the extracted nonlinear biascomponent.
 14. A computer readable recording medium storing a programfor executing a method of compensating for a bias applied to a head of ahard disk drive (HDD), the method comprising: measuring biases acrossall zones of a disk and extracting a linear bias component which islinearly changed across all zones of the disk and a nonlinear biascomponent which is nonlinearly changed across all zones of the diskbased on the measured biases, wherein the nonlinear bias componentcorresponds to a difference between the measured bias at each zone ofthe disk and a linear approximation of the biases across all zones ofthe disk; storing the linear bias component and the nonlinear biascomponent in a bias table; and compensating for a bias by referring tothe bias table when the HDD is turned on.
 15. A computer readablerecording medium storing a program for executing a method of editing abias table for compensating for a bias applied to a head of a hard diskdrive (HDD), the method comprising: measuring biases across all zones ofa disk and extracting a linear bias component which is linearly changedacross all zones of the disk and a nonlinear bias component which isnonlinearly changed across all zones of the disk based on the measuredbiases, wherein the nonlinear bias component corresponds to a differencebetween the measured bias at each zone of the disk and a linearapproximation of the biases across all zones of the disk; and editing abias table which stores the linear bias component and the nonlinear biascomponent.
 16. A hard disk drive (HDD) comprising: a disk which storesinformation; a spindle motor which rotates the disk; a head whichrecords information on the disk and which reads information from thedisk; a voice coil motor (VCM) which moves the head; a memory whichstores a bias table; and a controller which measures a bias in a trackand which compensates for the bias in a track seek mode by referring tothe bias table stored in the memory, wherein the memory stores a linearbias component which is linearly changed across all zones of the diskand a nonlinear bias component which is nonlinearly changed across allzones of the disk, wherein the nonlinear bias component corresponds to adifference between the measured bias at each zone of the disk and alinear approximation of the biases across all zones of the disk, and thecontroller compensates for the bias by referring to the bias table whenthe HDD is turned on, measures biases at reference positions on the diskwhen an operational condition of the HDD is changed, estimates a linearbias component changed across all zones of the disk based on biasesmeasured at the reference positions, and updates the linear biascomponent stored in the bias table to the estimated linear biascomponent.